Mussels and clams primarily differ in habitat and anatomy. Both mussels and clams are classified as bivalves and fall into the category of edible shellfish. Mussels congregate in freshwater and saltwater habitats, while clams are found in freshwater bodies such as lakes, ponds and streams.
Physically, mussels and clams have different body shapes and structure. Mussels are longer and narrower, and have shells that are longer and straighter. The bodies and shells of clams, by contrast, are usually circular or oval in shape, and have soft, rounded edges while the shells of mussels are linear and sharper. While the shells of clams function primarily as outer layers of clothing by giving clams a bit of protection from the elements, mussel shells serve a variety of practical purposes. These shells contain nutrients and minerals in their walls that provide sustenance for cells and tissues. While clam shells have one solid layer, mussel shells are made of three interconnected layers. The innermost shell layer is called the iridescent layer, followed by the prismatic layer, and lastly, the periostracum. While mussels have more complex outer shells, clams have more developed anatomical structures, complete with kidneys, hearts and mouths, and a more complex circulatory and respiratory system than mussels.